Microfluidic manipulation of nanometer- and micrometer-sized fluid-borne objects has become an important tool in the biological sciences. Some conventional manipulation techniques may employ a focused laser beam to create an optical trap behind a focus of the beam in which one or more objects may be held. Such systems are commonly referred to in the relevant art as “optical tweezers.”
Trapped objects can be used to exert forces on, or measure forces from, their local environments in a non-contact manner, with calibration performed using Hooke's Law. Optical traps have been used for quantitative observations of biological processes, examples of which include a motion of kinesin motor molecules and a force generated by RNA polymerase as it moves along a DNA molecule.
Performance of an optical trap depends on characteristics of the optical focusing element used to create the trap. In conventional optical tweezers, a laser beam typically is focused using a microscope objective lens. FIG. 1 shows an example of a conventional optical tweezers 100, in which a focused laser beam 101 passes through a large numerical aperture lens 102. To obtain a stiff three-dimensional optical trap, the conventional optical tweezers employ an oil or water immersion objective lens to produce an axial intensity gradient large enough to counter the scattering force from small angle rays. In the optical tweezers shown in FIG. 1, immersion oil 110 is placed between lens 102 and a coverslip 104. An object 106 suspended in water 108 is trapped in optical trap 105 which essentially coincides with the waist or focal spot of the laser beam 101 after passing through the lens 102.
A conventional high performance microscope objective lens typically employed in optical tweezers such as shown in FIG. 1 have extremely short working distances, e.g., from 100-1000 μm and usually less than 200 μm. Moreover, the microscope objective lens typically has a barrel width and length of 3 and 6 cm, respectively. Because of the large size and high cost of such lenses used in the conventional optical tweezers, they are generally considered impractical to integrate into microfluidic devices.